In recent years, fixed magnetic disc drives have been widely used as external memories for information processing devices, such as computers. FIG. 2 is a schematic cross sectional view showing the film structure of a typical magnetic recording medium known in the art. In the magnetic recording medium shown in FIG. 2, a non-magnetic layer 2 made of Ni--P or Al, for example, is formed on a substrate 1 made of Al or glass, for example, to provide a base 11, and a non-magnetic metal base layer 3, a hard magnetic layer 4, and a protective layer 5 containing carbon as a major component are successively formed in lamination on the base 11. Further, a lubrication layer 6 prepared from a liquid lubricant is formed on the protective layer 5. The above-indicated hard magnetic layer 4 consists of a single layer made of CoCrTa or CoCrPtTa, for example, and having a coercive force (Hc) of 0.13.times.10.sup.6 A/m or higher.
To produce the magnetic recording medium as described above, the non-magnetic substrate 1 made of an aluminum alloy or a glass material or the like is finished with desired parallelism, flatness and surface roughness, and the non-magnetic layer 2 consisting of Ni--P or Al film is formed on a major surface of the non-magnetic substrate 1 in a wet film-forming process, such as electroless plating, or a dry process, such as sputtering or deposition, to thus provide the non-magnetic base 11. Thereafter, the base 11 may be finished again by machining or reverse sputtering to achieve desired flatness and surface roughness. This base 11 is then heated to 150.degree. to 300.degree. C., and the non-magnetic metal base layer 3, hard magnetic layer 4 and protective layer 5 are successively formed on the surface of the base 11 while a dc bias of about 350V is applied to the base 11. The non-magnetic metal base layer 3 is made of Cr and has a film thickness of about 50 nm, and the hard magnetic layer 4 is made of a material, such as CoCrTa, which contains Co as a major component, and has a film thickness of about 30 nm, while the protective layer 5 contains carbon as a major component, and has a film thickness of about 10 nm. The protective layer 5 is then coated with a liquid lubricant containing fluorocarbon, which forms the lubrication layer 6 having a thickness of about 1 nm. In this manner, the magnetic recording medium 7 is produced.
In practical use, the magnetic recording medium thus produced exhibits excellent mechanical characteristics, such as high strength and high dimensional accuracy, and also exhibits excellent magnetic characteristics that the coercive force Hc is about 0.24.times.10.sup.6 A/m, and the product (Br.t) of the residual magnetic flux density and the film thickness is about 2.5.times.10.sup.-2 T.mu.m. The recording medium, however, has a relatively low coercive force angle ratio (S*) of about 0.75, that is the gradient of the magnetic curve near the coercive force Hc. As a technique for increasing the coercive force angle ratio (S*) while keeping the coercive force (Hc) constant in order to improve the above point, there is disclosed (in Digests of the 19th Annual conference on Magnetics in Japan 1995, 26aA-1)a method in which a Co alloy hard magnetic layer 40 as shown in FIG. 1 is separated into two layers, i.e., a first hard magnetic layer 4a formed in an Ar atmosphere, and a second hard magnetic layer 42 formed in an atmosphere that is a mixture of Ar and O.sub.2 gases.
With a rapidly increasing amount and variety of information in recent years, the fixed magnetic disc device is strongly desired to have a higher recording density and a larger capacity in view of the necessity for processing such a large amount of information. To this end, the magnetic recording medium is desired to have a high linear recording density, reduced noise (N), and good electromagnetic converting characteristics.
Further, there is a need to increase the percentage of non-defective (yield) so as to allow mass production of the magnetic recording medium at a reduced cost per product.